I've been playing this very small demo now for some 4 hours I am seriously surprised at how much fun the demo with it's limited amount of content already brings. There are 4 difficulty modes: Easy, Normal, Advanced and Expert. Now the first 3 won't pose any difficulties if you go only with any decent team of fps players, the first two being very easy and Advanced being rather easy. But Expert mode is really something else.
The Director, as Valve calls the AI engine, is a brilliant piece of engineering. If you think you can rest and camp out for 10 seconds somewhere the Director suddenly throws a raging horde of zombies at you. If it gets too easy it throws a Witch or 2 at you, and if it "feels" particularly nasty, the Director will throw a Tank at you. These babies look nasty in the downloadable movies, but they are bad when you're running around with all the hordes running around. Especially hard does it get when the Tank manages to take down 1 or 2 of your team mates. This boss really requires a team effort where there's strafing and shooting all around.
You can't be selfish in this game either, keeping health packs to yourself because if the team goes down the team goes down and you start the level all over again, with a total new challenge because the Director will spawn Bosses at different positions and responds to your reactions.
This is already a game I have no regrets in buying and I look forward to the full game in less than 2 weeks.